Many homes rely upon forced-air furnaces to provide heat during cool and/or cold weather. Typically, a forced-air furnace employs a burner that burns a fuel such as natural gas, propane or the like, and provides heated combustion gases to the interior of a heat exchanger. A circulating blower forces return air from the house over or through the heat exchanger, thereby heating the air. The combustion gases proceed through the heat exchanger to a collector box, and are then exhausted. In some cases, a combustion gas blower pulls the combustion gases through the heat exchanger and the collector box. The heated air is subsequently routed throughout the house via a duct system. A return duct system returns air to the furnace to be re-heated.
A gas valve controls how much fuel is provided to the burner. In some instances, a pressure drop across the heat exchanger, i.e., between the burner and the collector box, may be used as a signal to the gas valve to regulate gas flow to the burner, as this pressure drop is known to be at least roughly proportional to the combustion gas flow through the heat exchanger. However, this pressure signal is subject to transient spikes resulting from the combustion gas blower cycling on and off, system harmonics, and the like. Thus, a need remains for improved devices and methods of controlling furnaces such as forced-air furnaces.